Semiconductor devices of particular interest include switching elements for controlling current flow between two main electrodes, with current control involving the application of a control signal between a control electrode and one of the main electrodes. Among such devices are power transistors, MOSFET's (metal-oxide-semiconductor field-effect transistors), and IGBT's (insulated-gate bipolar transistors.)
To couple a main electrode or a control electrode formed on the surface of a semiconductor element to an external circuit, such electrode is connected to an output terminal via a relatively wide, belt-shaped insulated band conductor affixed to the same support plate as the semiconductor element. An electrode and a band conductor can be connected together by an ordinary lead wire, and band conductor and an output terminal can be connected together either by attaching the output terminal directly to the band conductor, or by means of a lead wire.
FIG. 2 shows a semiconductor device in which the semiconductor element forms an IGBT. In the case of FIG. 2, which does not use the present invention, (a) is a top view, (b) is a front view, and (c) is a side view. The IGBT semiconductor element 1 is affixed to a metal support plate 2 by means of a collector electrode on the bottom of the semiconductor element. On the surface of the metal support plate 2, a band conductor 41 is affixed on an insulating layer 3, and a band conductor 42 is affixed on an insulating layer which is not visible. An emitter main electrode 11 on top of the semiconductor element 1 is connected to the band conductor 41 by lead wire 51, and a gate control electrode 12 is connected to the band conductor 42 by lead wire 52. One end of the band conductor 41 rises up to form a main emitter terminal 6. Moreover, on the support plate 2, first and second auxiliary terminals 71 and 72 are affixed on an insulating layer 31. The first auxiliary terminal 71 is connected to the band conductor 42 by a lead wire 53, thereby forming a gate terminal, and the second auxiliary terminal 72 is connected to the band conductor 41 by a lead wire 54, thereby forming an auxiliary emitter terminal.
Motivated by recent demand for low-loss power switching elements, device operating resistance has been reduced by inclusion of a micro-pattern formed on the surface of a semiconductor element. As a further benefit of such inclusion, turn-on characteristics are enhanced. However, especially in cases of electric power handling, rapid turn-on is accompanied by a voltage surge, L.multidot.di/dt (where i stands for current, t for time, and L for the inductance of interconnections in the semiconductor device), and a voltage surge exceeding the rated voltage of an element may result in damage to the element. Such damage may also occur upon turn-off when, due to improved turn-off characteristics, di/dt is large in magnitude.
FIG. 3 shows an equivalent circuit for the semiconductor device of FIG. 2, where respective parts are numbered as in FIG. 2. When a positive voltage is applied to the gate terminal 71 and a negative voltage to the auxiliary emitter terminal 72, the IGBT element 1 is turned on, and the collector current 21 is proportional to the voltage between terminals 71 and 72. However, if the response speed of element 1 is too fast, the collector current 21 rises excessively. As a result, the voltage surge produced as the product of di/dt and the main-electrode interconnection inductance 22 becomes excessive. To prevent this, a resistor is connected in series to a gate terminal g, so that rise of a voltage between the gate electrode 12 and the emitter electrode 11 of element 1 is limited by a time constant which depends on the inherent capacitances 23 and 24 between the resistor and the element. This arrangement prevents the collector current from changing excessively upon start-up and shutdown. However, there are drawbacks in that a voltage signal reaching the gate is delayed, and in that the emitter voltage required to turn on a collector current 21 is large. Moreover, upon shutdown, the gate delay is long also, and the emitter voltage drop large.